The present invention relates to a diffraction element for use to diffract a laser light, which is emitted from a laser diode in an optical pickup device or the like, into three beams. More particularly, the invention relates to a diffraction element that can suppress or cancel a variation in the diffraction angle caused by a wavelength variation of the laser light that occurs with a temperature change.
A main stream of the DVD reproducing apparatuses is a so-called two laser type in which a 650 nm laser diode for reproducing the DVD and a 780 nm laser diode for reproducing/recording the CD-R are mounted to allow the CD-R to be reproduced. An optical pickup device mounted on such a reproducing apparatus is desirably made in smaller size and with lower price by reducing the number of optical members.
In recent years, a single lens system in which an objective lens is used commonly for both wavelengths has been established, and a so-called twin laser diode (Twin-LD) has been developed in which laser diodes having different wavelengths are integrated on a single semiconductor substrate, whereby the twin laser diode has been mounted as a laser light source of the optical pickup device. Also, for an optical element disposed between the laser light source and the objective lens, an optical element with wavelength selectivity for offering desired actions to two-wavelengths has been employed, instead of disposing a separate optical element for each wavelength.
For example, in connection with a diffraction element for producing three beams required for servo-controlling an optical spot position formed on an optical recording medium by the optical pickup device, diffraction elements provided for each wavelength was conventionally mounted on the optical system, but a diffraction element with wavelength selectivity (hereinafter referred to as a xe2x80x9ctwo-wavelength gratingxe2x80x9d in this specification) to provide the desired diffraction angle and diffraction efficiency for each of two-wavelengths has been recently employed.
Also, in order to detect a return light reflected from the recording face of the optical recording medium, a single detector which is commonly applicable for laser light beams of two-wavelengths has been recently employed. In this case, if the twin laser diode is mounted, laser light beams of two-wavelengths emitted from such laser light source will proceed in almost parallel, and is by no means converged at the same point in the single detector. Thus, a new diffraction element with wavelength selectivity (hereinafter referred to as an xe2x80x9coptical path composition elementxe2x80x9d in this specification) has been employed by changing the proceeding direction of a laser light of one wavelength to converge the laser light beams of two-wavelengths at the same point.
Herein, such diffraction element for use as the two-wavelength grating or optical path composition element is typically made of inorganic material with less thermal distortion. Such diffraction element has negligible thermal deformation caused by a change in the environmental temperature in the optical pickup device. In other words, because a grating pitch variation on the diffraction grating face due to thermal deformation can be ignored, the laser light of fixed wavelength can be diffracted at a desired angle, irrespective of temperature change.
However, a portion of the optical pickup device affected by a change in the environmental temperature is not only the optical element, but also the laser light source which is affected by heat. The laser light source has an oscillation wavelength fluctuated along with the change of the environmental temperature. As a result, the wavelength of the laser light incident upon the diffraction element is also fluctuated, and the diffraction angle is varied in accordance with the wavelength fluctuation. If the diffraction angle is varied, for example, in a case of the optical path composition element, such a trouble may arise that the laser light beams of two-wavelengths can not be converged at the same point.
For example, in a second laser light with a wavelength of 780 nm, the wavelength is varied from 784.7 nm to 793.6 nm, when the environmental temperature is changed from 20xc2x0 C. to 60xc2x0 C., resulting in a wavelength fluctuation of about 10 nm.
For example, the optical path composition element for diffracting a laser light with a wavelength of 780 nm as shown in FIG. 8 is exemplified. Five samples of the related optical path composition element made of inorganic material were prepared, and the change in the diffraction angle for each sample was measured when the environmental temperature was changed from 20xc2x0 C. to 60xc2x0 C. by every 10xc2x0 C. Thereby, the result was obtained as shown in a graph of FIG. 9. The diffraction angle for each of five samples was varied similarly to the theoretical values calculated from the grating pitch as indicated by the dashed line, whereby the diffraction angle was changed from 1.322 to 1.337 degrees when the environmental temperature was changed from 20xc2x0 C. to 60xc2x0 C. That is, the diffraction angle was changed by 0.015 degrees when the environmental temperature was changed from 20xc2x0 C. to 60xc2x0 C. This value corresponds to a coefficient of variation of 1.1% with respect to an diffraction angle of 1.322 degrees at an environmental temperature of 20xc2x0 C. In FIG. 8, the diffracted state at an environmental temperature of 20xc2x0 C. is indicated by the dashed line, and the diffracted state at an environmental temperature of 60xc2x0 C. is indicated by the solid line.
If this variation in the diffraction angle is represented by the displacement amount of a light beam spot converging on the light receiving face of the photo detector, the result is shown in a graph of FIG. 10. The displacement amount of light beam spot for five samples revealed the almost same change as the theoretical values as indicated by the dashed line that is calculated from the variation in wavelength. It was found that the spot position is moved 1.83 xcexcm when the environmental temperature was changed from 20xc2x0 C. to 60xc2x0 C.
Herein, the permissible range of spot displacement in the photo detector that is required in the typical optical pickup device is 2 xcexcm. Accordingly, there is no margin for the permissible displacement range at 70xc2x0 C. which is an upper limit of a guaranteed temperature range of the optical pickup device, resulting in lower yield of products.
This problem also arises with the laser light having a wavelength of 650 nm. Also, it likewise occurs in the two-wavelength grating.
In view of the above-mentioned problems, it is an object of the present invention to provide a diffraction element of wavelength selectivity that can suppress or cancel a variation in the diffraction angle caused by a laser wavelength variation that arises with a change in the environmental temperature.
In order to accomplish the above object, according to the present invention, there is provided a diffraction element comprising a diffraction grating formed with a grating pitch having different diffraction angles in accordance with a wavelength of a laser light beam incident thereon, the wavelength being variable in accordance with an environment temperature,
wherein the diffraction element is made of a material having a linear expansion coefficient which causes thermal variation in the grating pitch enough to compensate at least a part of the diffraction angle variation due to environment temperature change.
Herein, if the diffraction element of the invention is made of a material having a linear expansion coefficient in a range from 5xc3x9710xe2x88x925/xc2x0 C. to 3xc3x9710xe2x88x924/xc2x0 C., on the basis of the laser light wavelength (650 nm or 780 nm) for use with the typical optical pickup device and the permissible error required for the optical system, a variation in the diffraction angle caused by a wavelength variation that arises with a temperature change can be canceled by a grating pitch change due to thermal expansion or contraction of the diffraction element, to such an extent as to give rise to no practical trouble.
In the case where the diffraction element of the invention is employed as a two-wavelength grating in an optical pickup device, the diffraction characteristics may be set up such that the zeroth-order diffraction efficiency for one of the center wavelengths of 650 nm and 780 nm is 90% or more, and the first-order diffraction efficiency for the other is in a range from 10 to 20%.
In the case where the diffraction element of the invention is employed as an optical path composition element in an optical pickup device, it is preferable that the diffraction grating is a blazed diffraction grating in which each slant face is composed of a plurality of stepped faces. Here, it is preferable that each slant face includes 4 to 6 stepped faces.
Preferably, the diffraction characteristics may be set up such that the zeroth-order diffraction efficiency for one of the center wavelengths of 650 nm and 780 nm is 70% or more, and the first-order diffraction efficiency for the other is 50% or more.